Hue blend mode alters saturation too!

I have created a base image, with just stripes of pure red, green and blue, hsb 0,100,100 ; 120,100,100 ; 240, 100, 100.
I then added a color fill layer of pure blue, hsb 240, 100, 100.
When I set the blend mode to hue, I expected the whole image to turn blue, as I expected all the hues to go to 240 and the saturation and brighness to be unaffected, but I got saturations of 46 and 79 in the green and red areas respectively, giving a striped image.
Why was this? I thought the hue blend mode was supposed to affect hue only, and not affect saturation ???